Ribosome-inactivating proteins (RIPs) comprise a large group of toxic proteins widely distributed among the plant kingdom. RIPs are most active against nonplant, eukaryotic ribosomes, although activity against prokaryotic ribosomes has been reported. See, e.g., Stirpe et al., Biochem. J. 262:1001-1002 (1989). RIPs inactivate ribosomes by enzymatically attacking the 60S subunit of eukaryotic ribosomes and irreversibly modifying its large ribosomal RNA (rRNA). Barbieri et al., Cancer Surveys 1:129-141 (1982). This modification results from a specific RNA N-glycosidase activity that depurinates a single adenine found in a universally conserved loop of the large rRNA (A.sup.4324), See Endo et al., J. Biol. Chem. 262:8128-8130 (1987); Endo et al., J. Biol Chem. 262:5908-5012 (1987)).
Given the large number of RIPs described in the literature, relatively few RIP genes have been isolated and characterized. Characterization of the maize seed pro-RIP was first reported by Soave et al., Cell 27:403-410 (1981) prior to its identification as a RIP. EPO Publication No. 0 466 222 A1 discloses a RIP ("RIP1") produced in the kernels of maize in an inactive form which is activated by the cleaving of an internal peptide. The product is a two-chain active peptide moiety comprising a 16.5 kDa fragment of 169 amino acids and an 8.5 kDa fragment 107 amino acids; the dispensable region comprises 35 amino acids. The gene for the RIP1 maps to position 10 on the long arm of chromosome 8. The protein has been found only in the kernel and germinating seeds of maize. Bass et al., Plant Cell 4:225-234 (1992) disclose a cDNA of the RIP1 gene. The protein coded for is highly homologous to RIP1: it has a dispensable internal region; the first fragment varies from RIP1 by only 8 amino acids; and the second fragment is identical to RIP1.